Conventionally, a press brake having a construction as shown in FIG. 5 has been suggested as a kind of equipment for bending processing. In said drawing, the press brake has a bed 1 and a ram 2 which are oppositely arranged at the bottom and top. The equipment has side frames 3a, 3b in parallel, at both ends of the bed 1 and integral therewith in one piece, and hydraulic cylinders 4a, 4b are provided on the top ends of the side frames 3a, 3b. The bottom ends of cylinder axes 5, 5 of the hydraulic cylinders 4a, 4b are respectively connected with the left and right shoulders of said ram 2.
A table 6 is placed on said bed 1. A lower die (not shown) is set on the table 6 and an upper die (not shown) is set beneath the bottom end of the ram 2.
Thus for the bending processing, a work is inserted into the gap between the upper die and lower die and supported on the lower die, then both hydraulic cylinders 4a, 4b are operated to lower the ram 2, so the upper die presses the work on the lower die to bend it to the extent of a target bending angle.
FIG. 6 shows an appearance of the work 7 obtained as a result of the above-mentioned bending processing.
In said drawing, the bending angles of the work 7 are indefinite along its whole length, and the bending angles .theta.L, .theta.R at both ends of the work 7 are different from each other. Moreover, the bending angles .theta.L, .theta.R are not equal to the target bending angle .theta., either.
FIGS. 7 and 8 are prepared to show the factors which make the bending angles at both ends of a work different from each other.
In FIG. 7, a line C indicates the centerline of the machine and the work 7 is positioned being offset from this centerline to the right.
When the work 7 is bent under such all off-center condition, the side frame 3b to which the work is offset deforms larger than the other side frame 3a does. Because of this, the bending angles .theta.L, .theta.R at both ends of the work 7 fail to agree with each other.
In FIG. 8, numeral 8 shows the upper die fixed beneath the bottom of the ram 2 and numeral 9 shows the lower die fixed on the top of the bed 1. In the condition that the upper die 8 inclines to the right, the upper die 8 is fixed to the ram 2 with h1 (length of the upper die 8 projecting from the bottom end of the ram 2 at its right end) being larger than h2 (length of the upper die projecting from the bottom end of the ram at its left end).
If the die is incorrectly set like this, discrepancy arises between the bending angles .theta.1, .theta.2 at both ends of the work 7 as described above in the example shown in FIG. 7.